Resource
Also said as Currency, Economy
Anything a player has a limited amount of and can spend or use up to get something else: gold, ammunition, health, time on the clock, a turn. If it is scarce and you can choose how to spend it, it is a resource.
A choice only matters if it costs something. Nearly every interesting decision in a game comes down to which resource to spend, and with too much of everything there are no decisions left.
Say it
“Ammunition is not scarce enough to matter. Cut drops by 40%, and let players trade health items for bullets, so that shooting costs them something.”
Where you’ve seen it
Resident Evil gives you so little ammunition that each bullet is a question. StarCraft runs on two resources, minerals and gas, and needing both is what shapes every build.
Not to be confused with stat
A stat describes you and usually stays put, like strength. A resource goes down when you use it. Mana is a resource and intelligence is a stat.